1. Field of the Invention
The invention relates to an optical system for microlithographic projection exposure apparatuses, such as are used for the production of large-scale integrated electrical circuits and other microstructured components. The invention relates in particular to illumination systems having a plurality of reflecting surfaces for folding the beam path.
2. Description of Related Art
Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers to a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to a light of a particular wavelength range, for example light in the deep ultra-violet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of diffracting structures, which is arranged on a mask, is thereby projected onto the photoresist with the aid of a projection lens. Since the projection scale is generally less than 1, such projection lenses are often also referred to as reduction objectives.
The performance of such projection exposure apparatuses is determined not only by the imaging properties of the projection lens, but also by the properties of an illumination system which precedes the projection lens. Its object is to generate a projection light beam, which is directed at the mask to be projected. For this purpose the illumination system contains a light source, for example a pulse-operated laser, and a plurality of optical elements which generate a projection light beam with the intended properties from the light provided by the light source. These properties include, for example, the intensity and illumination-angle distributions over the cross section of the projection light beam, that is to say the angular distribution of the light rays which form the projection light beam.
Increasing importance is being attached to the polarization distribution of the projection light beam generated by the illumination system, since it has proved expedient for the projection light generated by the illumination system to arrive at the mask with a polarization state which is defined as accurately as possible. For example, it may be desirable for the projection light arriving at the mask to be linearly polarized substantially or fully in a given direction. Beam-splitter layers, which are often contained in catadioptric projection lenses, can achieve very high efficiencies with linearly polarized light in a suitable way. It may, however, be also desirable to direct substantially unpolarized or circularly polarized projection light onto the mask. For example, this can prevent or at least reduce resolution differences which depend on the direction of the structures contained in the mask.
The simplest situation is when the illumination system is intended to generate linearly polarized light, since the lasers conventionally used as light sources emit linearly polarized light. However, the subsequent optical elements can undesirably affect the polarization state of the projection light. If a rod homogenizer is being used for mixing the light, for example, then it is relatively difficult to control the polarization state of the projection light passing through. This is associated inter alia with the fact that skewed total reflections take place at the lateral interfaces of the rod homogenizer, and these change the polarization state by their phase-shifting effect. An undesirable effect on the polarization state can also be caused by diffractive optical elements with a polarization-dependent diffraction efficiency, or by optical elements which have an intrinsic or induced birefringence. The reflectivity of deviating mirrors, which may be arranged in illumination systems in order to fold the beam path, is also polarization-dependent.
Since the projection light is generally intended to arrive not only with an intended polarization at the mask but also with a defined polarization at the photoresist, attempts have also been made in the past to use measures inside the projection lens so as to affect the polarization of the projection light passing through. The context for this, inter alia, is that the projection lenses may also include optical elements which undesirably affect the polarization state of the projection light.